Assembly of single cells to form a diaphragm electrode unit

ABSTRACT

The invention relates to an assembly of flat single cells consisting of a lid polymer electrolyte and electrode areas applied to both sides thereof into a diaphragm electrode unit in which 2 to 10,000 single cells are connected in series through the stepwise overlapping of the electrode areas (4, 5, 6) of one single cell with the opposite electrode area (7, 8, 9) of the next cell, thus forming a one-dimensional diaphragm electrode unit (1), and a shunt conductive structure of electronically conductive material is placed at least between the overlapping electrode areas.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a U.S. national application of internationalapplication serial No. PCT/DE96/00111 filed Jan. 23, 1996, which claimspriority to German Serial No. 195 02 391.9 filed Jan. 26, 1995.

The invention relates to an assembly of single cells to form a diaphragmelectrode unit, with which the single cells, by stepwise superpositionof the electrode areas, are series-connected and to the use of same in apolymer electrolyte diaphragm fuel cell.

Electrochemical cells, e.g. with solid polymeric electrolytes (PEM),comprise, to put it simply, two electrodes on which the electrochemicalreactions proceed as well as an electrolyte situated therebetween,carrying out the task of transporting the ions between the electrodes,and being made of an ion-conductive polymer.

When electrochemical reactions voluntarily proceed on both electrodes(oxidation on the anode, reduction on the cathode), the electrochemicalcell will deliver a voltage. A single cell will deliver but a relativelylow voltage within the range of some millivolts up to some volts. Formany practical applications, such as e.g. application of battery fuelcells within traction range, substantially higher voltages are, however,needed.

That is why, up to now, a plurality of such cells have been separatelyassembled, arranged the one behind the other, and electrically connectedin series with one another so that the voltages of the single cells willbe added (bipolar stack-type construction). This way of bringing about aseries connection certainly enables a realisation of higher voltages;nevertheless, it gives rise to considerable drawbacks. For instance, theconstructional efforts of such a series connection are very great since,for one hydrogen/oxygen fuel cell stack, a bipolar plate, a hydrogen gasdistributing ring, an ion exchange diaphragm coated with a catalyser, anoxygen gas distributing ring, sealing rings for sealing those componentsas well as the current distributing structures are generally needed foreach single cell. This is a total of 10 components for each single cell.Now if, e.g., a stack output voltage of 70 V is to be realised, then, incase of a single cell voltage of 0.7 V, 100 single cells are requiredafter all, i.e. 1,000 components must be assembled, in which case 400sealing rings must be fixed.

Another drawback due to series connection is that, in case of failure ofonly one single cell in the fuel cell stack, the entire stack will breakdown. A redundant type of construction for the above example, i.e. aparallel connection of several stacks of 70 V, would, however, renderthe constructional efforts absolutely intolerable. Therefore, thedecisive factor for an efficient operation of a PEM fuel cellaccordingly is the structure of the diaphragm electrode unit.

Departing from these facts, it, thus, is the object of the presentinvention to specify a diaphragm electrode unit which is primarilysuited for use in PEM fuel cells and which shall possess a high outputvoltage as well as a simple and low-cost structure.

SUMMARY OF THE INVENTION

In accordance with the invention, it, thus, is suggested to assemble adiaphragm electrode unit such that an assembly of several single cellswill be provided, assembly being effected in such a manner that theelectrode areas will overlap stepwise and shunt conductive structuresbeing incorporated in the region of overlap.

The diaphragm electrode unit according to the invention comprisesionically conductive diaphragm districts that are bonded with electrodematerial on both sides. In case of a hydrogen/oxygen fuel cell, eachdiaphragm district will then be bonded with a hydrogen electrode on theone side and with an oxygen electrode on the opposed side. This beingso, all hydrogen electrodes are situated on one side of the diaphragmand all oxygen electrodes on the other side of the diaphragm. Togetherwith the bonded two electrodes, each diaphragm district constitutes afuel cell unit and, thus, delivers an output voltage. Now, thecharacteristic feature for the diaphragm electrode unit according to theinvention is that each single cell within the diaphragm isseries-connected. In accordance with the invention, this is broughtabout by realising, with a stepwise overlap of the electrode area of onesingle cell, a series connection with the opposed electrode area of thenext single cell. Thus, all the output voltages of each single cell areadded up. In this way, the sum of the voltages of all fuel cell units,e.g. on the first electrode of the upper side of the diaphragm and onthe last electrode of the lower side of the diaphragm, can be obtained.In case of the stepwise design according to the invention and belongingto the diaphragm electrode unit, it is important that a very good shuntconductivity of the individual external electrode areas be achievedsince all of the cell current must flow through the cross-section ofthis coating. However, it is known that electrodes, according to theirform of construction, e.g. when the electrode is made of a pressed-oncatalysing powder, show a poor shunt conductivity of up to several 100ohms. In order to avoid the high cell voltage losses resulting thereby,the diaphragm electrode unit according to the invention has so-calledshunt conductive structures that are located in the overlappingelectrode areas. The resistance generated by the poor shunt conductivityof the electrode coating and existing between two adjacent but opposedelectrodes will, thus, be discernibly reduced. This being so, adiaphragm electrode unit is made available for the first time, whichpossesses not only a high output voltage and a simple and low-coststructure but which also causes almost no internal cell voltage losses.

In order to ensure ion conductivity, the single cell, therewith, is madeof an ion conductive material. For this purpose, solid polymericelectrolytes in the form of diaphragms are used. Since it is eithercations or anions that must be transported, the diaphragm must bepermeable either to cations or to anions. In an aqueous environment forcation conductive polymers, ion conductivity is, therewith, generallygiven when the polymer includes firmly anchored carboxylic acid groupsand/or sulphonic acid groups and/or phosphonic acid groups, all ofwhich, so to say, are, in general, anchored by a chemical bond. Foranion conductive polymers, ion conductivity is given in particular whenthe polymer contains amino groups, quaternary ammonium groups, orpyridinium groups. The characteristic of ion conductivity is, in case ofthe hitherto described possibilities, brought about by ions which existand are firmly anchored in the diaphragm or which are produced in waterupon swelling.

Examples of cation conductive polymers of that kind are sulphonatedpolysulphones, polyether sulphones, or polyether ketones.

The thickness of the diaphragm may, therewith, be within the range offrom 0.1 μm to 5 mm and may, preferably, be within the range of from 10μm to 200 μm. The surfaces of the diaphragm for the single cell are,therewith, designed in dependency on the demanded performance of thestack. The surfaces may be within the range of from 100 μm² to 1,000,000mm² and may, preferably, be within the range of from 100 to 10,000 mm².

In order to enable functioning as a single cell, the above-describeddiaphragms now are coated with electrode material on both sides. Sincethe electrochemical reactions of the cell are effected on theelectrodes, the electrodes may either be properly made of that materialwhich is electrochemically reacted or they may be made of material whichcatalyses the electrochemical reaction. The material must beelectronically conductive and comprises in particular metals, metaloxides, mixed oxides, alloys, carbon, electronically conductivepolymers, or mixtures thereof.

The materials may contain additives serving for standardisation ofhydrophily and hydrophoby. With the aid of said additives, the electrodelayers may be equipped e.g. with water-repellent properties. Thematerials may, furthermore, contain additives which permitstandardisation of a certain porosity. This is of significanceespecially when gaseous agents are catalytically reacted on theelectrodes, a three-phase contact between gas, catalyser, and ionconductive district being required. Furthermore, so-called bondingagents may be admixed, making a stable and operative connection of theelectrode to the ion conductive district easier.

The shunt conductive structures must be made of materials which show avery good electronic conductivity. Metals, alloys, conductive carbons,conductive polymers, or polymers mixed with conductive substances aretypically used for this purpose. Thin structures of a thickness of from10 μm to 500 μm are preferably made use of since they can be wellintegrated into the surface structure of the diaphragm electrode unitaccording to the invention. Furthermore, the shunt conductive structuresshall, during cell service life, be stable towards the used fuels (e.g.water/oxygen in a H₂ /O₂ fuel cell) and shall not, with the occurringcell potentials, be electrochemically attacked. Apart from a goodconductivity, it is desired that the shunt conductive structures can bepermanently anchored in the strip diaphragm. That is why use ispreferably made of such structures that, with regard to their quality,do not show a smooth surface and, upon pasting or fusing, can be firmlyanchored with the diaphragm polymers. Such structures may be e.g. nets,tissues, porous structures, or foils with a roughened surface. Such aroughened surface can be brought about e.g. by means of chemicalprocesses or by plasma etching processes.

Now the shunt conductive structures may be so designed that they areguided up on to the external electrode areas. The shunt conductivestructures may, therewith, cover the electrode areas almost completelyor only partially.

According to a preferred form of construction, it is furthermoresuggested, and that for the case that the shunt conductive structurecovers the external electrode areas only partially, to additionallyarrange fuel-permeable, electronically conductive distributingstructures. Those conductive distributing structures may be arrangedeither directly on the electrode area or above the electrode area andthe shunt conductive structure. The task of said distributing structuresis to connect the shunt conductive structure to the entire electrodesurface without significant electric losses and to simultaneously enablea fuel supply to the electrode surface. Accordingly, the distributingstructures must likewise be made of an electronically conductivematerial. As examples thereof, metal nets or sintered metal compacts arementioned here.

The shunt conductive structures may also completely cover the electrodeareas. In this case, the shunt conductive structure, however, must, inthe region of the active electrode areas and in addition to electronicconductivity, also be fuel-permeable then since, otherwise, a supply ofthe electrodes with fuel would not be possible any more. Also for thisinventive form of construction, it is possible and advantageous to usedistributing structures yet which will then be arranged again in thearrangement of electrode surface/shunt conductive structure/distributingstructure or of electrode surface/distributing structure/shuntconductive structure.

The above-described assembly of flat single cells to form a diaphragmelectrode unit will lead to a one-dimensional arrangement.

In accordance with the invention, it now is also possible to assembleseveral ones of those one-dimensional diaphragm electrode units forforming "two-dimensional" diaphragm electrode units. Two forms ofconstruction are generally feasible herewith. On the one hand, at least2, and at most 50, one-dimensional diaphragm electrode units can beassembled, the latter being arranged in parallel and connected inparallel or connected in series.

By those forms of construction, a further increase in the output voltageand a redundant current supply, resp, are made possible.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features, details, and merits of the invention result from thefollowing description of the invention on the basis of the Drawing inwhich:

FIG. 1 shows a cross-section of a form of construction in accordancewith the invention, having shunt conductive structures guided up on tothe outer electrode areas;

FIG. 2 shows a plan view of the form of construction according to FIG.1;

FIG. 3 shows a cross-section of a form of construction, havingdistributing structures arranged on the shunt conductive structures;

FIG. 4 shows a cross-section of a form of construction, with which thedistributing structure is arranged between external electrode areas andthe shunt conductive structure, and

FIG. 5 shows a cross-section of a form of construction, with which theshunt conductive structure completely covers the external electrodeareas.

DETAILED DESCRIPTION OF THE INVENTION

According to the form of construction in FIG. 1, the diaphragm electrodeunit 1 consists of ionically conductive diaphragm districts 3 which arebonded with electrode material on both sides. In case of ahydrogen/oxygen/fuel cell, each diaphragm district 3 thus is, on the oneside, bonded with a hydrogen electrode 4, 5, 6 and, on the opposeddiaphragm side, with an oxygen electrode 7, 8, 9. All the hydrogenelectrodes 4, 5, 6, therefore, are situated on the one diaphragm sideand all the oxygen electrodes 7, 8, 9, thus, are situated on the otherdiaphragm side. Each diaphragm district, together with the two bondedelectrodes, constitutes a fuel cell unit and delivers an output voltageof about 1 volt without load. The inventive essence of the diaphragmelectrode unit 1 resides in that its internal single fuel cell units areconnected in series. For this purpose, the lower electrode 7, 8 of onefuel cell unit is connected with the upper electrode 5, 6 of the nextfuel cell unit through the conductive electrode material at a time, andthat in electronically conductive and fuel-impermeable manner. In thisway, the sum of the voltages of all fuel cell units on the firstelectrode 4 and on the last electrode 9 of the lower diaphragm side canbe obtained. In order to now achieve a good shunt conductivity of theelectrode areas 5, 6 and 7, 8, a shunt conductive structure 2 of goodelectronic conductivity is incorporated between the overlappingelectrode areas. According to the invention, it is sufficient in thiscase, when the shunt conductive structure 2 covers only the overlappingelectrode areas (marked by Symbol A). It is, however, preferred that theshunt conductive structure be passed through from the lower side to theupper side. The shunt conductive structure now passes therewith from oneelectrode area of one cell unit of the diaphragm electrode unit to theopposed electrode of the next cell unit of the diaphragm electrode unit.In this way, the electrode areas 5, 6 and 7, 8, resp, will, by means ofshunt conductive structures 2, be decisively enhanced as to their shuntconductivity. It is important to functioning that a good electronicconductivity of the shunt conductive structure be at hand. This isachieved by making use of correspondingly electronically conductivematerials. With the concept according to the invention, it isfurthermore essential that, upon guiding of the shunt conductivestructures 2 through the diaphragm electrode unit, there be no fuelpermeability from the one side of the diaphragm towards the other side.

Such a diaphragm electrode unit is made from polymeric solid electrolytepieces coated with electrode material, one shunt conductive structurebeing placed between two solid electrolyte pieces at a time, whichstructure extends from the lower side of the respectively first solidelectrolyte piece towards the upper side of the second solid electrolytepiece. These arrangements, each consisting of solid electrolytepiece/shunt conductive structure/solid electrolyte piece, willsubsequently be durably and fuel-tightly connected with one another.Connection of the solid electrolyte pieces with one another and with theshunt conductive structures may be effected e.g. by means of pastingtechniques with suitable adhesives.

FIG. 2 now shows a plan view of the above-described form of constructionand makes it clear once again that, in the form of constructionaccording to FIG. 1, the shunt conductive structure 2 overlaps theexternal electrode areas 5, 6 only partially.

In the form of construction according to FIG. 3, the shunt conductivestructure 2 is now directly placed upon the individual electrodes 5, 6and 7, 8, resp. The shunt conductive structure 2 may, therewith, be madeof dense material or of nets, provided that good electrode conductivityis given and that transport of fuels from the one towards the otherstrip diaphragm side is prevented. In addition to all that, afuel-permeable, electronically conductive distributing structure 10 is,in the form of construction according to FIG. 3, placed upon theelectrode area with the shunt conductive structure 2, making it its taskto electrically connect the shunt conductive structure, withoutsignificant losses, to the entire electrode area and to enable a fuelsupply to the electrode surface at the same time.

FIG. 4 now shows another form of construction, and that a variant, withwhich the fuel-permeable, electronically conductive distributingstructure 10 is placed upon the electrode area 4, 5, 6 and 7, 8, 9,resp, and the shunt conductive structure 2 is arranged on the structure10 only in this case. This arrangement is advantageous in that theelectrode surface will be evenly mechanically loaded whereas, with theform of construction according to FIG. 3, the end of the shuntconductive structure 2 will be placed directly upon the electrode areaso as to be pressed into the diaphragm when the cell is being assembled.Also with the form of construction according to FIG. 4, the shuntconductive structure 2 may be made e.g. of dense material or of nets,provided that good electron conductivity is given and transport of fuelsfrom the one to the other side is prevented.

The shunt conductive structures may also completely cover the electrodeareas. Such a form of construction is shown in FIG. 5. In this case, theshunt conductive structure 11 must, in addition to being electronicallyconductive, also be fuel-permeable in the region of the active electrodeareas since, otherwise, a supply of the electrodes 5, 6 and 7, 8, resp,with fuel would not be possible any more. But also with that form ofconstruction, additional use of the distributing structures as describedabove is possible so that here then an arrangement of electrodearea/shunt conductive structure/distributing structure or of electrodearea/distributing structure/shunt conductive structure will be feasible.

We claim:
 1. Assembly of flat single cells, each being made up of asolid polymeric electrolyte and of electrode areas applied to both sidesthereof to form a diaphragm electrode unit wherein 2 to 10,000 singlecells are, by stepwise overlap of the electrode areas (4, 5, 6) of onesingle cell, connected in series with the opposed electrode areas(7,8,9) of the next single cell and thus constitute a one-dimensionaldiaphragm electrode unit (1) and wherein, at least between the electrodeareas overlapping one another, a shunt conductive structure (2, 11) madeof an electronically conductive material is arranged.
 2. Assembly ofsingle cells according to claim 1, wherein the conductive structure (2,11) is guided up on to the external electrode areas while partiallyoverlapping same.
 3. Assembly of single cells according to claim 2wherein the shunt conductive structure (2, 11) is guided up on to theexternal electrode area, said shunt conductive structure (2, 11) almostcompletely covering that electrode area.
 4. Assembly of single cellsaccording to claim 2 wherein, on the external electrode areas and theshunt conductive structure, a distributing structure (10) is arranged soas to be at least partially fuel-permeable.
 5. Assembly of single cellsaccording to claim 2 wherein, between the external electrode areas andthe shunt conductive structure (2, 11), a distributing structure (10) isarranged so as to be at least partially fuel-permeable.
 6. Assembly ofsingle cells according to claim 2 wherein the electronically conductivematerial of the shunt conductive structures (2, 11) is selected frommetals, alloys, conductive carbon modifications, conductive polymers,and mixtures thereof.
 7. Assembly of single cells according to claim 1wherein the shunt conductive structure (2, 11) is guided up on to theexternal electrode area, said shunt conductive structure (2, 11) almostcompletely covering that electrode area.
 8. Assembly of single cellsaccording to claim 7 wherein, on the external electrode areas and theshunt conductive structure, a distributing structure (10) is arranged soas to be at least partially fuel-permeable.
 9. Assembly of single cellsaccording to claim 7 wherein, between the external electrode areas andthe shunt conductive structure (2, 11), a distributing structure (10) isarranged so as to be at least partially fuel-permeable.
 10. Assembly ofsingle cells according to claim 7 wherein the electronically conductivematerial of the shunt conductive structures (2, 11) is selected frommetals, alloys, conductive carbon modifications, conductive polymers,and mixtures thereof.
 11. Assembly of single cells according to claim 1wherein, on the external electrode areas and the shunt conductivestructure, a distributing structure (10) is arranged so as to be atleast partially fuel-permeable.
 12. Assembly of single cells accordingto claim 11 wherein, between the external electrode areas and the shuntconductive structure (2, 11), a distributing structure (10) is arrangedso as to be at least partially fuel-permeable.
 13. Assembly of singlecells according to claim 11 wherein the electronically conductivematerial of the shunt conductive structures (2, 11) is selected frommetals, alloys, conductive carbon modifications, conductive polymers,and mixtures thereof.
 14. Assembly of single cells according to claim 1wherein, between the external electrode areas and the shunt conductivestructure (2, 11), a distributing structure (10) is arranged so as to beat least partially fuel-permeable.
 15. Assembly of single cellsaccording to claim 14 wherein the electronically conductive material ofthe shunt conductive structures (2, 11) is selected from metals, alloys,conductive carbon modifications, conductive polymers, and mixturesthereof.
 16. Assembly of single cells according to claim 1 wherein theelectronically conductive material of the shunt conductive structures(2, 11) is selected from the group consisting of metals, alloys,conductive carbon modifications, conductive polymers, and mixturesthereof.
 17. Assembly of single cells according to claim 1 wherein theshunt conductive structures (2, 11), are of a thickness of from 0.1 to 5mm.
 18. Assembly of single cells according to claim 1 wherein the shuntconductive structures (2, 11) are a structure having a non-smoothsurface, from the group consisting of nets, tissues, porous structures,and structures having a roughened surface.
 19. Assembly of single cellsaccording to claim 1 wherein the distributing structure (10) is anelectronically conductive structure.
 20. Assembly of single cellsaccording to claim 1 wherein at least 2 and at most 50, one-dimensionaldiaphragm electrode units are arranged in parallel and connected inseries.
 21. Assembly of single cells according to claim 1 wherein atleast 2 up to a maximum of 50 one-dimensional diaphragm electrode unitsare arranged in parallel and connected in parallel.
 22. An assembly ofsingle cells according to claim 1 wherein the assembly is used in adiaphragm polymer electrode cell.